1. Field of the Invention
The disclosure relates to an all-fiber-based laser system. The disclosure relates to an all-fiber-based laser system which can actively control passive mode locking.
2. Description of the Related Art
A pulse fiber laser has potential to be used in applications such as; machining of solid and brittle material, in medical examinations and in wavelength transformation. Maximum peak power output is mainly provided by an active nanosecond fiber laser and a passive mode locked laser. However, the above two lasers have the following disadvantages. The peak power of the active nanosecond fiber laser is not provided efficiently enough for machining of solid and brittle material. Furthermore, because the pulse duration is longer than a picosecond or a femtosecond of a short pulse, a great deal of heat is generated. The passive mode locked laser is easily affected by environment, is costly and can not be actively modulated, thus, it is not available for laser machining.
A conventional all-fiber-based high peak power nanosecond pulse laser uses a master oscillator power amplifier, MOPA. The MOPA with a seed and a multi-stage amplifier, and a plurality of light isolators are assembled to form a high peak fiber laser. However, when the pump light power is out of proportion to the seed power, the pump light is emitted to an Yb doped gain fiber, and a non-directive amplified spontaneous emission, ASE is generated. The ASE is suppressed to prevent the laser from proceeding along a former path to damage the seed. Because the all-fiber-based laser is serially connected via melt, prevention of ASE is more important.
Outer shell electrons of the Yb doped fiber are easily ionized and raised to a upper energy level and fast decay to meta-stable state. If the seed power is not enough, the power of the pump light increases, and disturbs the gain fiber, a spontaneous pulse is generated in a instant short time. The pump light can not enter the disturb bent fiber in a transient state. When gain fiber is interfered, energy is provided for the fiber laser to generate a spontaneous pulse in a very short time. The pulse repetition frequency is related to the lifetime of the Yb doped fiber. The lifetime of the Yb doped fiber is around 850 μsec.
FIG. 1 is a schematic view of a conventional mode locked fiber laser. Referring to FIG. 1, FIG. 1 is U.S. Pat. No. 7,317,740 “MODE LOCKER FOR FIBER LASER”. The mode locked laser includes a laser unit 100, a mode locker 104, two collimators 104a and 104c, a pillar-shaped structure 104b, a rotator structure 104d, a light coupler 106, a gain fiber 102, and a wavelength division multiplexer 108. The laser unit 100, the mode locker 104, the light coupler 106, the gain fiber 102 and the wavelength division multiplexer 108 are connected in sequence. The mode locked laser is not an all-fiber-based laser, and the mode locker 104 is adjusted by a mechanism.
FIG. 2 is a schematic view of a conventional mode locked fiber laser. Referring to FIG. 2, FIG. 2 is U.S. Pat. No. 7,477,664 “Nonlinear Polarization pulse shaping mode locked fiber laser”. The mode locked fiber laser 200′ comprises a wavelength division multiplexer 210′, an Yb doped fiber 205, a coupler 230, a fiber output 225, two polarization controllers 204-1′ and 240-2′, a linear polarization isolator 235′. The wavelength division multiplexer 210′, the Yb doped fiber 205, the coupler 230, the fiber output 225, the polarization controller 240-2′, the linear polarization isolator 235′, and the polarization controller 204-1′ are connected in sequence. However, the mode locked fiber laser 200′ must be switched to lock mode by a mechanism, and the mode locked fiber laser 200′ is sensitive to the environment, thus, the mode locked fiber laser 200′ is unstable. Furthermore, the mode locked fiber laser 200′ does not actively modulate.
FIG. 3 is a schematic view of a conventional mode locked fiber laser. Referring to FIG. 3, FIG. 3 is U.S. Pat. No. 6,097,741 “Passive mode-locked fiber lasers”. A fiber laser 300 includes a first reflector 310 with a grating 312 and a collimator 314, a gain material 330, a fiber coupler 340, a fiber 342, three fiber portions 340a, 340b and 344, two sections 364 and 362, a pump light 350, a fiber output coupler 360, a light isolator 370, a collimator 321, a lens 323, a saturable absorber 325 and a second reflector 326. This design is very complicated and needs many optical elements. The fiber laser 300 must be adjusted to lock mode by mechanism, and the fiber laser 30 is sensitive to the environment, thus, the fiber laser 30 is unstable. Furthermore, the fiber laser 30 does not actively modulate.